Britain was pulled closer towards a renewed cold war with Russia when David Cameron announced UK military trainers are to be deployed to help Ukraine forces stave off further Russian backed incursions into its sovereign territory.

The decision – announced on Tuesday but under consideration by the UK national security council since before Christmas – represents the first deployment of British troops to the country since the near civil war in eastern Ukraine began more than a year ago.

Downing Street said the deployment was not just a practical bilateral response to a request for support, but a signal to the Russians that Britain will not countenance further large scale annexations of towns in Ukraine.

The prime minister said Britain would be “the strongest pole in the tent”, and argued for tougher sanctions against Moscow if Russian-backed militias in eastern Ukraine failed to observe the provisions of a ceasefire agreement reached this month with the Ukrainian president, Petro Poroshenko.

Downing Street said some personnel would be leaving this week as part of the training mission. Initially 30 trainers will be despatched to Kiev with 25 providing advice on medical training, logistics, intelligence analysis and infantry training. A bigger programme of infantry training is expected to follow soon after taking the total number of trainers to 75.

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They will not be sent to the conflict zone in eastern Ukraine.

Personnel involved in the training elements could spend one or two months in the country, with a command and control deployment lasting up to six months. Britain has previously offered some ad hoc military training inside Ukraine but that was before the conflict sent relations between the EU and Russia into the deep freeze.

The US already has some military training underway.

Cameron’s decision, made to a Commons committee of MPs, is bound to worsen Anglo-Russian relations , but the prime minister, who has been accused of losing influence in Europe and sleepwalking into the conflict over Ukraine, insisted he did not see a military solution to the crisis. He also said he did not support at this stage sending lethal equipment to Kiev, an issue that is still under consideration in Washington.

Cameron said Europe listened to British expertise on how to implement sanctions, and said the level of sanctions “might become materially different” in the future. He held out the possibility of excluding Russia from the Belgian-based international Swift banking payments system, saying there was a logic to such a move if Moscow continued trying to “dismember” Ukraine.

Swift is a secure messaging system used by more than 10,500 banks for international money transfers. If deprived of access to the system, Russian banks and their customers couldn’t readily send or receive money across the country’s borders, which could potentially disrupt trade, investment, and millions of routine financial transactions

Cameron said: “I think what we should be putting in to place is a sense that if there is another Debaltseve (a strategic Ukrainian railway town seized by pro-Russia forces this month) then that will trigger a round of sanctions that will be materially different to what we have seen so far,” he said.

He also pointed to a new tripwire saying: “People will be looking at Mariupol as the next potential flashpoint, and if that were to happen I think the argument for further action would be overwhelming. I think that would be the view of countries like Poland, the Baltic states and many others.” Mariupol, a huge steel town, is coming closer to the frontline.

After a ceasefire deal struck in Minsk on 15 February, fighting in eastern Ukraine has decreased in intensity but shows little sign of easing in some key areas.

Pro-Russia rebels said on Tuesday they had started pulling heavy weaponry back from the frontline in accordance with the putative peace plan, which aims to create a buffer zone between the two sides’ artillery.

But the claims were rejected as “mere empty words” by the Kiev military, which accused the separatists of “making use of the ceasefire period” to build up ammunition, Reuters reported.

It said one of its soldiers had been killed and seven wounded in the previous 24 hours, and reiterated that it would not start pulling back weapons until shooting stopped.

Dismissing suggestions Britain had been an onlooker in the crisis, he told MPs Britain “should not feel precious” about whether the UK was in the room in every negotiation over Ukraine. In recent weeks German chancellor, Angela Merkel, and the French president, François Hollande, have been in direct talks with Vladimir Putin to try to broker a ceasefire, reinforcing the impression that a Franco-German motor is at the heart of EU foreign policy.

Cameron said there would be “deeply damaging” consequences for all of Europe if the EU failed to stand up to Putin on Ukraine, predicting that the Russian president could turn against the Baltic states or Moldova if he was not reined in now.

He suggested it would be “miraculous” if the terms of the 12 February ceasefire agreement brokered in Minsk by Merkel and Hollande were met in full.

He said there was no doubt about Russian support for the rebels. “What we are seeing is Russian-backed aggression, often these are Russian troops, they are Russian tanks, they are Russian Grad missiles. You can’t buy these things on eBay, they are coming from Russia, people shouldn’t be in any doubt about that.

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“We have got the intelligence, we have got the pictures and the world knows that. Sometimes people don’t want to see that but that is the fact.”

He added: “If there was major further incursion by Russian-backed forces and effectively Russian forces into Ukraine, we should be clear about what that is. That is trying to dismember a democracy, a member of the United Nations, a sovereign state on the continent of Europe, and it’s not acceptable.”

Although the defence secretary, Michael Fallon, had earlier highlighted Russian air incursions along the Channel, Cameron produced figures to play down suggestions that the Russians were stepping up pressure on the UK.

He argued that “because a couple of Russian planes fly around the Channel we shouldn’t talk ourselves into a situation where we think somehow we cannot defend ourselves. We absolutely can.”